A conventional method of ocular administration of aqueous solutions or suspensions comprises the use of eye drops. This is generally known to have low patient acceptability, especially in the young, and it is necessary, for administration, to incline the recipient's head towards a horizontal position. The administration of a large drop of liquid to the eye initiates a blink reflex, which can result in a substantial wastage of the applied liquid or suspension by drainage either through the tear ducts or onto the skin surface. Indeed, it has been reported that if a 30-50 .mu.l drop is applied to the eye, the actual volume that remains at the target is only 5-7 .mu.l. Therefore, in addition to the low patient acceptability, there is a 4-to 10-fold wastage. This leads to inefficiency in the use of expensive ingredients and, in addition, the administrator has little control, and is uncertain, over the amount of liquid which actually reaches the target. This is particularly important if the liquid is a solution or suspension of an ophthalmologically-active therapeutic substance.
Another conventional method of ocular administration of an ophthalmologically-active therapeutic substance comprises the use of an ointment. This similarly has been found to have low patient acceptability and, in this method also, a substantial wastage of active ingredient can result.
These problems in the efficient administration of therapeutically active substances to the eye are largely overcome in European Patent No. 0 224 352B by generating a spray of electrically charged droplets of a liquid formulation comprising an ophthalmologically-active substance and an ophthalmologically-acceptable diluent, for subsequent administration to the eye. The formulation has a viscosity in the range 10.sup.-3 to 1.0 Pa.s at 25.degree. C., and a resistivity in the range 10.sup.4 to 10.sup.12 ohm.cm at 25.degree. C. The formulation is applied to a spray nozzle wherein a sufficiently large electrical potential relative to earth is applied to the formulation from a high voltage generator, that sufficient electrical gradient is produced at the nozzle to atomize the formulation as a spray of electrically charged droplets.
Although such a method allows the delivery to the eye of an optimum small volume of a formulation of a therapeutic substance, without requiring the recipient's head to be inclined towards the horizontal, it does, however, still have some drawbacks. Solutions or suspensions containing more than about 50% of water, that is, of lower resistivity than 10.sup.4 ohm.cm, cannot be sprayed, and high voltages of 15 kV or higher are used. Further, an electrode needs to be in contact with the formulation, to achieve the correct voltage for atomization, and this could cause cross-contamination problems for pharmaceutical formulations. A further disadvantage is that a formulation containing substantial amounts of non-aqueous solvents, which is to be dispensed by this method, is likely to be hypertonic, which although acceptable for very low volume applications can result in a stinging sensation if larger volumes are administered to the eye.